This invention relates to centrifugal pumps of the kind having a shrouded impeller and a single entry eye, the impeller being rotatable in a casing the interior of which is subjected to the pressure generated by the pump. In such a pump, the impeller is subjected to an axial thrust because of the following: The effective axially-projected front area of the intake eye is unbalanced with respect to the fluid pressure upon it, namely the mean intake pressure (or "suction"), which acts on the upstream or front side of the impeller only. The fluid pressure within the casing acts on the axially projected area of the shroud to result in an axial thrust on the front of the impeller; in the opposite direction, this fluid pressure acts on the back of the impeller over the whole of its projected area. Such pumps are known (and are the particular kind to which the invention applies) in which the impeller rotor unit embodies an armature having a spherically convex surface corresponding to the spherically concave surface of a thin non-magnetic wall of the casing, there being a small gap between such surfaces. The rotor unit is driven as an induction motor by the electromagnetic field of coil windings external to the said wall. For example, centrifugal pumps of this type are described in U.S. Pat. No. 3,354,833. The magnetic forces tend to thrust the rotor unit axially rearward, but such thrust is not always sufficient to exceed the net axial thrust due to the fluid pressure referred to above.
When the impeller of such a pump is pivotably supported by an axial bearing, the combined effect of the fluid pressure and magnetic thrusts may be such--especially when running in a throttled condition--as to lift the impeller rotor unit away from such bearing. This is a condition that could destroy the pump.
The main aim of the invention is to ensure, in such a pump, that under all conditions of operation, the impeller rotor is held in engagement with the bearing.